Make verify stage jobs manual

What does this merge request do and why?

This MR makes the verify:* stage manual by default but can be trigged by starting the pre-verify:verify-begin job while we work through improving the speed of verify:* jobs and improve the turnaround time for MRs where not every MR needs all verify:* jobs to be executed.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.